The mode switch is likely not getting vacuum or has a break in the vacuum control hose(s). When the mode switch has zero vacuum, the default is to send air to defrost vent at glass. And, I am assuming you have the engine running. There are a number of reasons for the issue, like a vacuum canister that is leaking, lines cut (or chewed on by critters), disconnect of vacuum source at engine, etc. I do not know if yours is a class A or C with a Ford chassis, or not.
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